How to properly connect the Denoise node in Compositor?
When in doubt, use the Image pass.
Image

provides unmodified render result
output also comes denoised by the Internal denoiser if it's enabled


Noisy Image

provides a Despeckled image (useful when render has fireflies)
may cause bad shadow artifacts for low sample counts


Further Reading:
Denoise node in Blender Manual
How to use Intel denoiser in Blender
Blenderartists.org discussion from Aug 2019
intel openimage denoise